1. Field of the Invention
The present invention relates to a semiconductor light modulator which can control an amount of light transmission by applying a prescribed voltage to an active layer sandwiched between two clad layers of different conductivity types.
1. Description of the Background Art
A small semiconductor light modulator capable of low voltage operation and having a small wavelength variation (chirping) is widely used in a large capacity optical communication system employing an optical fiber. The large capacity optical communication system is becoming more popular in these days as the Internet becoming more widespread, and there is a great need to enhance a response speed of the semiconductor light modulator.
In general, a reduction of a device capacity is essential to enhance a response speed (a bandwidth) of a light modulator using a semiconductor. In a conventional semiconductor light modulator, a low voltage operation and a high extinction ratio are obtained by applying a voltage to a light absorption layer as an active layer, utilizing a Franz-Keldysh effect in a bulk absorption layer and utilizing a quantum confined Stark effect in a multiple quantum well (MQW) structure.
Commonly, a so-called p-i-n structure is widely used as a semiconductor light modulator, which is constructed by sandwiching a light absorption layer with semiconductor layers respectively including a p-type impurity and an n-type impurity from upside and downside. In order to reduce a device capacity in this semiconductor light modulator, it is necessary to increase the thickness or to decrease the width of the light absorption layer.
Increased thickness of the light absorption layer, however, causes a higher operation voltage and, moreover, a connection property with an optical fiber will be degraded (an insertion loss will be increased) because an amount of confined light in the light absorption layer will increase as the light absorption layer with a high refractive index becomes thicker. In addition, an extinction ratio will become lower when the width of the light absorption layer is decreased. Therefore, it is impossible to obtain a semiconductor light modulator which satisfies all of the required properties, that is, an enhanced response speed (a broadband (a reduced capacity)), a high extinction ratio, a low voltage operation, and a low insertion loss.
In references such as Japanese Patent Laying-Open Nos. 61-212823, 8-248363, and Proc. IPRM 2001 pp. 151-154 WP-06, a semiconductor light modulator having a structure different from that mentioned above is proposed to realize the semiconductor light modulator which can satisfy all those properties. In contrast with the conventional semiconductor light modulator which has the p-i-n structure formed in a stacking direction, the semiconductor light modulator described in these references has the p-i-n structure formed in a horizontal direction perpendicular to the stacking direction, which enables an individual control of a device capacity and a spot size of light to satisfy all the required properties, i.e., a low voltage operation, a high bandwidth, a low insertion loss, and a high extinction ratio.
A problem still remains, however, in the semiconductor light modulator having the p-i-n structure formed in a horizontal direction as described in the above-mentioned references. Generally, in a semiconductor light modulator, an InGaAsP bulk or an MQW (Multiple Quantum Well) structure is used in a light absorption layer, and InP is used in a clad layer. As InP and InGaAsP have different band gap energies, there is a high hetero barrier between the light absorption layer and the clad layer. Because of this hetero barrier, carriers (holes or electrons) generated within the light absorption layer especially during a high light input pile up near the hetero barrier, which degrade the response speed of the semiconductor light modulator.
An object of the present invention is to provide a semiconductor light modulator having an enhanced response speed by suppressing a pile-up of carriers due to a hetero barrier.
A semiconductor light modulator according to a first aspect of the present invention includes a semi-insulating substrate, a first conductivity type clad layer formed on the semi-insulating substrate, a second conductivity type clad layer having a conductivity type different from that of the first conductivity type clad layer and formed on the semi-insulating substrate in the same height position as the first conductivity type clad layer aligned with a prescribed space, an active layer provided between the first conductivity type clad layer and the second conductivity type clad layer, and a band discontinuity reduction layer provided between the active layer and the first conductivity type clad layer and having a band gap energy larger than that of the active layer and smaller than that of the first conductivity type clad layer.
According to the above-mentioned structure, the pile-up of carriers can be prevented by the band discontinuity reduction layer provided between the active layer and the first conductivity type clad layer and having a band gap energy of a magnitude between that of the active layer and that of the first conductivity type clad layer. As a result, the response speed of the semiconductor light modulator can be enhanced.
More preferably, the semiconductor light modulator according to the first aspect of the present invention includes a buffer layer provided between the active layer and the band discontinuity reduction layer and having a thickness enabling a tunneling of an electron or a hole at a voltage during use. The buffer layer is formed of a material that enables the band discontinuity reduction layer to be formed directly on the buffer layer.
According to the above-mentioned structure, when the band discontinuity reduction layer cannot be formed directly on a surface of the active layer in the above-mentioned structure, the band discontinuity reduction layer can easily be formed by forming in advance the buffer layer between the active layer and the band discontinuity reduction layer and then forming the band discontinuity reduction layer on a surface of the buffer layer.
In addition, the above-mentioned buffer layer preferably has a thickness of 0.1 xcexcm or less. This suppresses the disadvantage of the pile-up of carriers caused by the buffer layer due to the excess thickness of the buffer layer.
A semiconductor light modulator according to a second aspect of the present invention includes a semi-insulating substrate, a first conductivity type clad layer formed on the semi-insulating substrate, a second conductivity type clad layer having a conductivity type different from that of the first conductivity type clad layer and formed on the semi-insulating substrate in the same height position as the first conductivity type clad layer aligned with a prescribed space, and an active layer provided between the first conductivity type clad layer and the second conductivity type clad layer. The active layer is a multiple quantum well active layer having a barrier layer and a well layer, and includes a mixed crystal portion of the barrier layer and the well layer near the surface of the active layer that faces the clad layer.
According to the above-mentioned structure, the pile-up of carriers can be prevented because the mixed crystal portion functions as the band discontinuity reduction layer. As a result, the response speed of the semiconductor light modulator can be enhanced.
A semiconductor light modulator according to a third aspect of the present invention includes a semi-insulating substrate, a first conductivity type clad layer formed on the semi-insulating substrate, a second conductivity type clad layer having a conductivity type different from that of the first conductivity type clad layer and formed on the semi-insulating substrate in the same height position as the first conductivity type clad layer aligned with a prescribed space, and an active layer provided between the first conductivity type clad layer and the second conductivity type clad layer. The active layer includes an impurity of the first conductivity type near its surface that faces the first conductivity type clad layer.
According to the above-mentioned structure, the region near either side of the hetero junction is doped to be the first conductivity type. In other words, the hetero junction is surrounded by the region doped with the first conductivity type impurity. Therefore, the band discontinuity barrier becomes lower so that holes or electrons can pass over the discontinuity barrier more easily. As a result, the response speed of the semiconductor light modulator can be enhanced.
A semiconductor light modulator according to a fourth aspect of the present invention includes a semi-insulating substrate, a first conductivity type clad layer formed on the semi-insulating substrate, a second conductivity type clad layer having a conductivity type different from that of the first conductivity type clad layer and formed on the semi-insulating substrate in the same height position as the first conductivity type clad layer aligned with a prescribed space, and an active layer provided between the first conductivity type clad layer and the second conductivity type clad layer. The first conductivity type clad layer includes the same substance as a main component of the active layer as a main component.
According to the above-mentioned structure, the response speed of the semiconductor light modulator can be enhanced by lowering the hetero barrier and suppressing the pile-up of carriers (holes or electrons) as compared with a structure in which the first conductivity type clad layer includes a substance different from a main component of the active layer as a main component.
The features of the respective semiconductor light modulators according to the first through fourth aspects mentioned above may be combined with each other.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.